Petrol bombs and other missiles were hurled at police by angry crowds during sporadic violence, leaving 129 officers injured.

The Police Service of Northern Ireland (PSNI) has also been addressing the severe dissident republican risk, unrest surrounding loyal order parades and planning for major events surrounding the UK City of Culture in Londonderry and the World Police and Fire Games in Belfast later this year.

Mr Baggott told MPs at Westminster: "We have to look forward to the next three years, the relentless pressure that is going to apply on us, the current trajectory of the dissident threat and potentially what that means and whether we are sufficiently resilient."

Senior officers are preparing a report on how many extra members they think are needed for the 7,000-strong force - half what it was at the height of the troubles - but warned policing was only part of the solution to communal enmity.

Terry Spence, chairman of the Police Federation which represents rank and file members, said: "There has been a headlong dash for normality in Northern Ireland which has seen a hasty and invidious run down of the number of police officers available for the maintenance of the peace."

The force was shrunk following a review of policing in Northern Ireland carried out by Lord Patten, which recommended a reduction in numbers to reflect the relative peace in the region. This approach has long been questioned by the Federation, a view drawing support from many unionists but opposed by most nationalists who supported the complete overhaul of the force and the replacement of many seasoned "old guard" officers with Catholic new recruits.

Since then, dissidents have carried out a number of high-profile killings and there have been annual riots over loyal order parades during the summer marching season - with eight weeks of flag-related demonstrations the latest challenge to confront the service.

Some loyalists are angry at Belfast City Council's decision to only fly the emblem from the City Hall on designated days like royal birthdays and that anger has turned ugly, with sledgehammers, fireworks and even a railway sleeper used to attack police lines in small parts of Northern Ireland. Police blamed loyalist paramilitaries for much of the recent trouble.